let's say i don't want to run a c3 network but am using c3-equipped units. is there a flat multiplier i can apply to reduce a force's PV?
No. It depends on the offensive versus defensive components. the C3 only affects the offensive side.
It's still usually only a single point or two.
WHM-10CT is 38 without C3S, 40 with
WHM-8D drops from 43 to 41.
BLR-2Dr Warlord drops 2 as well.
White Flame 3B drops 2.
Wight drops 1.
WFT-B drops 1. (27 to 26).
So maybe a typical 1/20 (round down) less. (I tried a 33 and 38 PV and they each only dropped 1 as well). But the WVR-9W2 only dropped 1 from 40 to 39. More of it's PV comes from defense/movement and less from offense (compared to WHM-10CT that was also 40, but dropped to 38). But the Wraith dropped only 1 (from 40), but the Yao Lien dropped 2 (from 39 to 37).
So, no, there's no accurate flat multiplier. But if it's a single 'mech or two, you're talking 1-3 PV total you're paying for C3 and not using. And if you have 4+, then make a c3 network! :)